Promoting Active Species Generation by Electrochemical Activation in

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Promoting Active Species Generation by Electrochemical Activation in Alkaline Media for Efficient Electrocatalytic Oxygen Evolution in Neutral Media Kun Xu, Han Cheng, Linqi Liu, Haifeng Lv, Xiaojun Wu, Changzheng Wu, and Yi Xie Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04732 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 4, 2016

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Promoting Active Species Generation by Electrochemical Activation in Alkaline Media for Efficient Electrocatalytic Oxygen Evolution in Neutral Media Kun Xu‡a, Han Cheng‡a, Linqi Liua, Haifeng Lvb, Xiaojun Wuab, Changzheng Wua*, Yi Xiea a

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China

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CAS Key Laboratory of Materials for Energy Conversion and Department of Material Science and Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China

ABSTRACT In this study, by using dicobalt phosphide nanoparticles as precatalysts, we demonstrated that electrochemical activation of metallic precatalysts in alkaline media (comparing with directly electrochemical activation in neutral media) could significantly promote the OER catalysis in neutral media, specifically realizing a two-fold enhanced activity and meanwhile showing a greatly decreased overpotential of about 100 mv at 10 mA cm-2. Comparing with directly electrochemical activation in neutral media, the electrochemical activation in harsh alkaline media could easily break the strong Co-Co bond and promote active species generation on the surface of metallic Co2P, thus account for the enhancement of neutral OER activity, which is also evidenced by HRTEM and the electrochemical double-layer capacitance measurement. The activation of electrochemical oxidation of metallic precatalysts in alkaline media enhanced neutral OER catalysis could also be observed on CoP nanoparticles and Ni2P nanoparticles, suggesting this is a generic strategy. Our work highlights that the activation of electrochemical oxidation of metallic precatalysts in alkaline media would pave new avenues for the design of advanced neutral OER electrocatalysts. KEYWORDS Metallic precatalysts, electrochemical activation, generic strategy, enhanced neutral OER activity

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Hydrogen generation by electrolysis of water provides an attractive way for renewable energy conversion and storage.1-4 In general, the process of water electrolysis is composed of two half reactions: cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER).5,6 However, the process of water electrolysis usually undergoes high overpotential and notable efficiency loss mostly due to the notorious sluggish anodic OER process, arising from multiple proton coupled electron transfer.7-9 To improve the efficiency of electrolysis of water to produce hydrogen, exploring highly efficient OER electrocatysts for promoting the anodic OER process is critical. Recently, significant efforts have been made to develop effective OER electrocatalysts and a series of Co- and Ni-based compounds have shown great potential as electrocatalysts for OER in alkaline conditions.10-17 Nevertheless, the electrolysis of water under the alkaline conditions is not environmentally friendly and economic. In contrast, electrolysis of water under neutral conditions is considered benign and could reduce the cost of electrochemical water splitting system, which would not require expensive membranes either.18,19 In this regard, it is highly desirable but challenging to develop neutral OER electrocatalysts. During the past few years, various transition metal-based materials have been widely studied as promising electrocatalysts for electrochemical water oxidation under neutral conditions.20-24 For example, 3D hierarchical cobalt hydroxide carbonate hydrate directly grown on FTO glass substrate has exhibited capability to catalyze oxygen evolution reaction under neutral pH conditions.23 Moreover,


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Hydrated Manganese(II) Phosphate was also reported as a high-efficiency anode electrocatalyst for water splitting under neutral pH.24 However，most of these reported transition metal-based materials suffer from poor electrical conductivity, which may impede the electron transport inside the catalysts bulk component and thus inhibit their catalytic efficiency.25 With that in mind, metal non-oxide based compounds with great conductivity may be good candidate materials for catalytic water oxidation under neutral conditions. Unfortunately, for most of metal non-oxide compounds such as nitrides, phosphides and carbides, there are numerous strong metal-metal bonds in the crystal structure. In this case, forming the electrocatalytic active species (metal oxide or hydroxide) on the surface of these precatalysts is quite insufficient in the directly electrochemical activation process, as it is difficult to break strong metallic bond and generate active species in temperate neutral medium. Thus, although the great conductivity could ensure the rapid electron transport inside the electrocatalyst bulk component which is favorable for OER process, the less density of active species forming on the surface of metal non-oxide based compounds still impede the neutral OER catalytic process. As stated above, forming active species on the surface of metal non-oxide compounds through direct activation of electrochemical oxidation in neutral media is not enough for the neutral OER catalytic process due to the abundant metal-metal bonds in metal non-oxide compounds. To further enhance the OER catalytic efficiency of metal non-oxide based compounds under neutral conditions, it is necessary to increase the density of active species on the surface of metal non-oxide


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based compounds. Herein, we highlight that activation of electrochemical oxidation of metallic precatalysts in alkaline media could trigger more efficient electrocatalytic oxygen evolution in neutral media, comparing with that of direct activation of electrochemical oxidation in neutral media. Specially, we select Co2P as the model neutral OER precatalyst due to its intrinsical metallic state (Figure S2). The temperature dependence of electrical resistance was performed to reveal metallic nature of Co2P sample, and the metallic behavior would facilitate electron migration between surfaces of the catalysts and electrode (Figure S4). The electrochemical measurements show that the activation process of Co2P nanoparticles at different conditions contribute different OER activity under neutral condition. Our studies indicate that the harsh alkaline environment could greatly facilitate the formation of active species on the surface of metal non-oxide precatalysts, finally allowing more efficient water oxidation under neutral conditions. More interestingly, this strategy is general and also effective for CoP nanoparticles and Ni2P nanoparticles. Our work provides an insight into the activation of metal non-oxide based compounds for OER catalytic process in neutral media. Co2P nanoparticles were synthesized by a simple colloidal method.26-28 Firstly, the phosphorous source trioctylphosphine (TOP) and octadecene served as reductive agents which could reduce the cobalt source Co(acac)2 to Co (0). Then, the Co2P nanoparticles were prepared by Co further reacting with TOP (Figure S6).29 The X-ray diffraction (XRD) was performed to study the structural information of as-prepared samples. As shown in Figure 1a, all peaks presented in the XRD pattern


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of as-prepared samples could match well with orthorhombic-Co2P phase (JCPDS Card No. 32-0306; Space group: Pnam(62); a=5.647 Å; b=6.61 Å;c=3.513 Å), suggesting the formation of Co2P. To investigate the morphology and microstructural information of the as-prepared Co2P, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed. As shown in Figure 1b, the TEM image demonstrates the Co2P nanoparticles have a uniform size distribution with a diameter of 20±5 nm. The HRTEM image in Figure 1c shows a lattice spacing value of 0.210 nm which could ascribe to (211) plane of Co2P. Furthermore, the HAADF-STEM and EDX elemental mappings (Figure 1d) of Co2P nanoparticles were also carried out, in which we can find that the Co and P elements are homogeneously distributed throughout the particles. Therefore, the results above clearly demonstrate that the Co2P nanoparticles were successfully controllable-prepared. In the light of the latest research results of our group and colleagues, the sufficient metal oxo-/hydroxide active species exposed on the surface of metal non-oxide are necessary for the OER catalytic process.25, 30 On this occasion, for neutral OER process, the direct activation of electrochemical oxidation of metallic Co2P (denoted as n-Co2P) will not be able to generate enough of the active species on the surface because of abundant strong Co-Co metallic bonds in metallic Co2P. Thus, we choose the activation of electrochemical oxidation for Co2P nanoparticles in the harsh alkaline electrolytic beforehand (denoted as a-Co2P) to ensure enough active species forming on the surface of Co2P nanoparticles. To evaluate the neutral OER


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activity of n-Co2P and a-Co2P, the electrochemical measurements were performed in O2-saturated neutral electrolyte of 0.1M PBS solution. It needs to be noted that all of the OER polarization curves were recorded after dozens of CV cycles until the CV curves were stable. Obviously, as shown in Figure 2a, the a-Co2P shows a smaller neutral OER onset overpotential (~0.32 V) than that of n-Co2P (~0.37 V). Meanwhile, the a-Co2P also shows a greatly decreased overpotential of about 100 mv at 10 mA cm-2 compared with that of n-Co2P (1.822 V for a-Co2P and 1.924 V for n-Co2P), approaching to precious metal oxide IrO2 electrocatalyst. Figure 2b exhibits the OER current densities with a-Co2P almost the double of the n-Co2P at a constant overpotential under neutral conditions. Furthermore, the a-Co2P also exhibits a smaller Tafel slope value of 129.8 mV dec-1, while n-Co2P possess a Tafel slope value of 168.9 mV dec-1 (Figure 2c). As well known, the small Tafel slope values of electrocatalysts are favorable for practical utilizations because it will give rise to an intensive increment of OER rate with overpotentials. To gain further insight into the neutral OER reaction kinetics, the electrochemical impedance spectroscopy (EIS) was also performed (Figure 2d). The frequency range of EIS is from 100 kHz to 0.1 Hz at a potential of 1.8 V vs. RHE and the corresponding amplitude is 5 mV. Nyquist plots shows that the equivalent circuit is made up of a solution resistance (Rs), a constant phase element (CPE) and a charge transfer resistance (Rct). It unravels that the charge transfer resistance (28.2 Ohm) of a-Co2P catalyst is significantly lower than that of n-Co2P catalyst (62.5 Ohm), indicating the a-Co2P catalysts own the faster charge transfer process during the neutral OER catalysis process. Noting that the solution


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resistance (Rs) of a-Co2P is smaller than that of n-Co2P, the difference of Rs in the same electrolyte may originate from different contact resistance between electrode and electrolyte. Compared with n-Co2P, the a-Co2P is more hydrophilic due to more metal oxo-/hydroxide active species exposure on Co2P surface, giving rise to the smaller contact resistance between the electrode and electrolyte. Together with above OER polarization curves Tafel slope data and EIS results, we can safely draw the conclusion that the electrochemical activation of Co2P in alkaline media (comparing with that of directly electrochemical activation in neutral media) could significantly promote the OER catalytic process in neutral media. It must be noted that，apart from Co2P, the activation of electrochemical oxidation of other metallic precatalysts (such as CoP and Ni2P) in alkaline media could also significantly enhance neutral OER activity, indicating the generality of our strategy (Figure S14). As well known, stability is also a key index to evaluate electrocatalysts. In our case, we used accelerated degradation studies and galvanostatic testing to assess the stability of the a-Co2P nanoparticles. Figure 3a shows the accelerated degradation studies of a-Co2P nanoparticles. The overpotential for achieving 5 mA cm-2 only increases 5 mV after 1000 CV cycles, indicating the high stability of a-Co2P nanoparticles as high-performance neutral OER electrocatalysts. Furthermore, continuous OER at different static overpotential (chronoamperometry measurement) was also performed. As shown in Figure 3b, the OER current density of a-Co2P nanoparticles at different overpotential could be maintained over 20 h without significant degradation, also suggesting the great stability of a-Co2P nanoparticles for


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neutral OER electrocatalysis process. The Faradaic efficiency by a fluorescence-based oxygen sensor (Ocean Optics) in Figure S9 exhibits a Faradaic yield of ∼95% for oxygen production after electrolysis for 3500s, showing superior Faradic efficiency for a-Co2P nanoparticles. Therefore, all these results above illustrates that the a-Co2P nanoparticles own excellent OER activity as well as great durability in neutral media. According to Nernst equation, the surface of Co2P is easier to generate cobalt oxo-/hydroxide active species in alkaline media than in the neutral media. To confirm and understand that the electrochemical activation of Co2P nanoparticles in harsh alkaline media could promote active species generation on the surface of metallic Co2P, which account for the enhancement in neutral OER activity, the “after-testing” HRTEM and EDX mapping were carried out. Figure 4a shows HRTEM image of n-Co2P after neutral OER catalytic process and amorphous cobalt oxo-/hydroxide layer of 2 nm was observed around the surface of Co2P nanoparticles. In contrast, a thicker amorphous layer of 4-5 nm on the surface of Co2P was observed on a-Co2P after neutral OER catalytic process (Figure 4b). The only difference between a-Co2P and n-Co2P is that the one is activated in alkaline media and the other is in the neutral media. Therefore, the different layer thickness of cobalt oxo-/hydroxide active species was originated from the harsh alkaline media that facilitates the active species forming on the surface of metallic Co2P nanoparticle. To further understand the amorphous layer on the surface of Co2P nanoparticles, EDX elemental mappings and line scan were also recorded in Figure 4c and 4d. The results clearly demonstrate Co and P distribute homogenously throughout Co2P nanoparticles while O has a dense


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distribution around Co2P nanoparticles. Furthermore, X-ray absorption near-edge structure (XANES) measurements were performed to show local structure and chemical configuration of catalyst surface (Figure 4e). The XANES profile of cobalt exhibits a pre-edge peak in pristine Co2P nanoparticles, which is ascribed to the cobalt alloy. For a-Co2P after OER, this feature becomes weaker and an energy shift is observed on the rising edge. This indicates the formation of cobalt oxo-/hydroxide on the surface of Co2P and the chemical valence of cobalt becomes higher. The Fourier transforms of the k3-weighted EXAFS oscillations (FT-EXAFS) at Co K-edge of the Co2P nanoparticles and a-Co2P after OER were shown in Figure 4f. The spectra show a radial distribution function (RDF) of the R-3c phase. The peaks of Co2P NPs at about 2.10 Å and 2.40 Å are corresponding to the Co-P and Co-Co bonds. It needs to be noted that the EXAFS spectra show no significant change on Co-P and Co-Co bonds after the formation of cobalt oxo-/hydroxide on the surface of Co2P, which illustrates that no obvious bulk phase transformation occurred. The Co-P peak at 2.1 Å shows a little decrease and a slight low-r shift of 0.05 Å. It is attributed to the formation of cobalt oxo-/hydroxide on the surface of Co2P, due to the shorter Co-O bond length and the weaker photoelectron scattering amplitude of the O atom. The Co-O peak at 1.70 Å was directly observed due to the formation of cobalt oxo-/hydroxide on the surface of Co2P. All the results illustrate that the formation of active species phase on the surface of Co2P were cobalt oxo-/hydroxide. Moreover, the electrochemical double-layer capacitance measurements were also performed to investigate the electrochemical active surface area of the n-Co2P and a-Co2P (Figure


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S16). The value of Cdl of n-Co2P is about 49.09 µF cm-2, which is significantly smaller than that of a-Co2P (90.72 µF cm-2). The results also illustrated that the activation of electrochemical oxidation in harsh alkaline media could facilitate the formation of cobalt oxo-/hydroxide layer on the surface of metallic Co2P, which provides more active species exposure on the surface of metallic Co2P. Therefore, it is reasonable that the electrochemical activation of Co2P in alkaline media realizes enhanced OER activity under neutral conditions, comparing with that of Co2P directly electrochemical activation in neutral media. In summary, we have demonstrated that activation of electrochemical oxidation of metallic precatalysts in alkaline media could trigger more efficient electrocatalytic oxygen evolution in neutral media, comparing with that of precatalysts direct activation of electrochemical oxidation in neutral media. Taking metallic Co2P nanoparticles as an example, upon activation of electrochemical oxidation under alkaline conditions, the a-Co2P catalysts show a significant decrease of about 100 mV overpotential at 10 mA cm-2 while showing a two-fold enhanced activity in neutral media, comparing with that of n-Co2P direct activation of electrochemical oxidation in neutral media. As evidenced by HRTEM analysis and EDLC results, this improvement strongly relies on promoting active species generation on the surface of metallic precatalyst by electrochemical activation in alkaline media. Furthermore, we show that this strategy is general and effective to other metallic precatalyst such as CoP and Ni2P for neutral water oxidation, which was also evidenced. We believe that our work could provide new insights for understanding the active phase for OER and


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open a new door to design of advanced electrocatalysts in neutral media.

ASSOCIATED CONTENT Supporting information Experimental section, additional figures, analysis and discussion. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author To whom correspondence should be addressed. E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank Prof. Wangsheng Chu and Mr. Hui Xie at USTC for the help of XAFS experiments and analyses. This work was financially supported by the National Basic Research Program of China (2015CB932302), Natural Science Foundation of China (No. U1432133, 21331005, 21601172, U1532265, J1030412), National Program for Support of Top-notch Young Professionals, Chinese Academy of Science (XDB01020300), the China Postdoctoral Science Foundation (Grant No. 2015M580539, 2016T90571) and the Fundamental Research Funds for the Central Universities (No. WK2060190027, WK2060190061). REFERENCES (1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. USA 2006, 103, 15729-15735.


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(24) Jin, K.; Park, J.; Lee, J.; Yang, K. D.; Pradhan, G. K.; Sim, U.; Jeong, D.; Jang, H. L.; Park, S.; Kim, D.; Sung, N.-E.; Kim, S. H.; Han, S.; Nam, K. T. J. Am. Chem. Soc. 2014, 136, 7435-7443. (25) Xu, K.; Ding, H.; Lv, H.; Chen, P.; Lu, X.; Cheng, H.; Zhou, T.; Liu, S.; Wu, X.; Wu, C.; Xie, Y. Adv. Mater. 2016, 28, 3326-3332. (26) Ha, D.-H.; Moreau, L. M.; Bealing, C. R.; Zhang, H.; Hennig, R. G.; Robinson, R. D. J. Mater. Chem. 2011, 21, 11498-11510. (27) Brock, S. L.; Perera, S. C.; Stamm, K. L. Chem.– Eur. J. 2004, 10, 3364-3371. (28) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Angew. Chem. 2014, 126, 5531-5534. (29) Jin, Z.; Li, P.; Xiao, D. Green Chem. 2016, 18, 1459-1464. (30) Zhang, Y.; Ouyang, B.; Xu, J.; Jia, G.; Chen, S.; Rawat, R. S.; Fan, H. J. Angew. Chem. Inter. Ed. 2016, 55, 8670-8674.


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FIGURES

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Figure 1. (a) XRD patterns for as-prepared Co2P nanoparticles. (b) TEM image of as-prepared Co2P nanoparticles. (c) HRTEM image of as-prepared Co2P nanoparticles. (d) The HAADF-STEM and EDX elemental mapping of Co2P nanoparticles.


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Figure 2. (a) The polarization curves of commercial IrO2, n-Co2P nanoparticles, a-Co2P nanoparticles and bare GC. (b) Comparison of the OER current density of n-Co2P nanoparticles and a-Co2P nanoparticles at different overpotential. (c) Tafel plots for as-obtained n-Co2P nanoparticles and a-Co2P nanoparticles. (d) Nyquist plots of n-Co2P nanoparticles and a-Co2P nanoparticles with the equivalent circuit (inset).


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Figure 3. (a) Polarization curves of a-Co2P nanoparticles before and after 1000 cyclic voltammetry cycles. (b) Chronoamperometry of n-Co2P nanoparticles at different overpotential.


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Figure 4. (a) HRTEM image of n-Co2P after neutral OER catalysis process. (b) HRTEM image of a-Co2P after neutral OER process. (c) HAADF-STEM and EDX elemental mapping images of a-Co2P after neutral OER process. (d) EDX line scan of the single a-Co2P nanoparticle after neutral OER process with a corresponding to the arrow in Figure 4c. (e) XANES profile of Co2P nanoparticles (red) and a-Co2P nanoparticles after neural OER process (blue). (f) FT-EXAFS spectra at Co K-edge of Co2P nanoparticles (red) and a-Co2P nanoparticles after neural OER process (blue).


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Table of Contents


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Promoting Active Species Generation by Electrochemical Activation in

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